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In article <>, says...
> Hi, does anyone know how I could get a 555 timer to operate in astable
> mode at a 50% duty cycle? All the circuits I've seen are always for
> greater than 50%. thanks.
>
>
I just googled 555 50% PWM and got :http://www.dprg.org/tutorials/2005-11a/index.html

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James Beck wrote:
> In article <>,
> says...
>
>>Hi, does anyone know how I could get a 555 timer to operate in astable
>>mode at a 50% duty cycle? All the circuits I've seen are always for
>>greater than 50%. thanks.
>>
>>
>
> I just googled 555 50% PWM and got :
> http://www.dprg.org/tutorials/2005-11a/index.html
>
> Look at what is done with the diodes D1 and D2
>
> Jim
>
Also notice that the timing resistor is fed from the Output and the Load
fed from the Discharge pin (pullup resistor required or directly drive
a load with low side switching). If you dont need a variable duty
cycle, leave out the diodes and try a single timing resistor from the
Output to the junction of Threshold and Trigger. It will probably be
very close to 50% but might be off by a little. The circuit Jim gave is
trimmable.

On 27 Dec 2006 10:59:18 -0800, "panfilero" <>
wrote:
>Hi, does anyone know how I could get a 555 timer to operate in astable
>mode at a 50% duty cycle? All the circuits I've seen are always for
>greater than 50%. thanks.

>> mode at a 50% duty cycle? All the circuits I've seen are always for
>> greater than 50%. thanks.
>>
>>
> I just googled 555 50% PWM and got :
> http://www.dprg.org/tutorials/2005-11a/index.html
>
> Look at what is done with the diodes D1 and D2
>
> Jim

The CMOS version of the 555 (e.g. 7555) is far superior to the Bipolar
version (e.g. NE555) for a variety of reasons. Reason 1 is reduced power
consumption. Reason 2 is the absence of the quirky short circuit spike (400
ma) during an output transition. Reason 3 is that output levels of the CMOS
version approach the supply rails (ground or common and VCC) as they should.
Also I believe the voltage divider resistors in the bipolar version are 1K
while they are 100K or more in the CMOS version. This is very useful if
you're modulating the pin 5 voltage level.

Hooking the output of the 555 (pin 3) to the RC network will give you a 50%
duty cycle as John mentioned and this always works best with the CMOS
version.

panfilero wrote:
> Hi, does anyone know how I could get a 555 timer to operate in astable
> mode at a 50% duty cycle? All the circuits I've seen are always for
> greater than 50%. thanks.

Connect a signal diode like 1N4148 in parallel with the resistor
between pins 6 and 7, with the cathode (stripe) oriented toward pin 6.
With this arrangement you can get any duty cycle you want. You can
even get a fixed frequency, variable duty cycle oscillator if you
replace the fixed resistors with potentiometer. Connect the ends of
the pot to pins 6 and 8, the wiper to pin 7, and the diode from pin 7
to pin 6.

"James Beck" <> wrote in message
news:...
> In article <>,
> says...
> > Hi, does anyone know how I could get a 555 timer to operate in astable
> > mode at a 50% duty cycle? All the circuits I've seen are always for
> > greater than 50%. thanks.
> >
> >
> I just googled 555 50% PWM and got :
> http://www.dprg.org/tutorials/2005-11a/index.html
>
> Look at what is done with the diodes D1 and D2
>
> Jim
>
If you have the space, feed the output of the 555 into the clock input of an
edge-triggered J-K FF. Tie J and K high to create a toggle and the output
will be a nice 50% duty cylce at 1/2 the input frequency.

On 2006-12-27, panfilero <> wrote:
> Hi, does anyone know how I could get a 555 timer to operate in astable
> mode at a 50% duty cycle? All the circuits I've seen are always for
> greater than 50%. thanks.

change the voltage on pin 5 or use a different circuit, how much precision
do you need?